I am working on a matrix library. At the moment I am trying to write a function that will display the given matrix in the console.
However I don't really have any good idea how to get started. I mean, which stream should I use (for example iostream or stringstream), how to make formatting in details?

The main requirements are:
1) Display the numbers in given precision in fixed format, but if the length of resulting output in fixed format is too big, than convert (only this matrix element) to scientific format. (therefore I hesitate if I should use stringstream, as it easily alllows to check the length of the resulting output)

2) There should exist easy way of redirecting the stream to file.

3) It should be possibly efficient, however this is not the main goal

And other thing: should I buffer the output and then print the whole thing, or better to cout element after element. Won't be using stringstream for formatting, and then using cout for printing too slow?

I really need some general directions (but code snippets are very welcome too).

If you want to make your output function have a flexible choice of destination, you can make it take a std::ostream & argument for destination. Then whoever uses it can dump it to a std::stringstream, std::cout or a std::fstream. One option is to just overload std::ostream & operator<<(std::ostream &, const YourClass &) so you can use the formatted output operator.

If you want to make your output function have a flexible choice of destination, you can make it take a std::ostream & argument for destination. Then whoever uses it can dump it to a std::stringstream, std::cout or a std::fstream. One option is to just overload std::ostream & operator<<(std::ostream &, const YourClass &) so you can use the formatted output operator.

This. And for requirement (1) I'd wrap the sprintf() call to make a fancy formatting (sorry for non C++ approach, but I don't know any decent STL classes to solve this issue).

I'm not sure why you worry about performance here. Writing to an I/O stream and displaying stuff on a terminal will always be slow, no matter how efficient your code is. Clearly this is a debugging feature, right? Also, do you want a human-friendly output, like:

The problem with the human-friendly output is that it spans multiple lines, which doesn't quite fit the concept of string (sure, you can use newlines, but you need to add them everywhere to make sure the matrix doesn't come out garbled because it started at the end of an existing line, etc...), whereas with the parser-friendly output, you can print it easily to the console or a file, possibly with a debug label, and then feed them to Mathematica or Wolfram|Alpha or even LaTeX (perhaps using a script) which'll display type-setted matrices to you (and you can even then perform computations on them to check them), but of course this can be less practical depending on your needs.

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

If you want to make your output function have a flexible choice of destination, you can make it take a std::ostream & argument for destination. Then whoever uses it can dump it to a std::stringstream, std::cout or a std::fstream. One option is to just overload std::ostream & operator<<(std::ostream &, const YourClass &) so you can use the formatted output operator.

This. And for requirement (1) I'd wrap the sprintf() call to make a fancy formatting (sorry for non C++ approach, but I don't know any decent STL classes to solve this issue).

I'm curious why this was down-voted? There are no C++ functions that provide formatting like the old C functions. This has led me on many occasions to prefer the C functions over C++ or to mix them. If the person that down-voted this has a C++ solution, please post it.

There are no C++ functions that provide formatting like the old C functions.

Can you give an example of formatting that you can do with the C standard library functions that you can't do with the C++ standard library functions? I can't think of any (though I will freely admit that the C++ versions are much, much more annoying to use).

You can, for example, set the number of digits to be printed after the decimal in a floating point number. You can set the number of leading/trailing zeros in a number. You can print anything in hexadecimal without writing a conversion function. You can choose to print in decimal or scientific notation. The list goes on. I've searched, and I cannot find this ability in any C++ stream. Maybe I haven't searched well enough?

I usually like my matrices and vectors in Matlab-Syntax, such that I can just copy & paste them into Matlab/Octave. I feel that if the matrix is simple, I can look at it in the debugger, and if it is complicated, then actually looking at it won't do any good and further computations in Matlab/Octave are necessary anyways.

I know, this is rather unrelated to you question, but I thought I should suggest it, if only as an extra option.